300: Rise of an Empire (2014),
which was ill-known at the time by its wooden dialogue and stilted speeches and the gore-filled action sequences, had been considered as a mild action movie by today’s standards.
Option Elimination -
A. which was ill-known at the time by its wooden dialogue and stilted speeches and the gore-filled action sequences, had been considered as - in contemporary English, the better usage would be "little-known" than "ill-known." Moreover, "known for" is the better idiom structure. As this is a subjective/hypothetical assessment based on the current standards, it is better to use "would be" than the past perfect passive "had been considered." Also, another way of looking at the conditional or hypothetical would be "if the movie were judged by today's standards, it would be considered a mild action movie." 2nd Conditional.
B. ill-known at the time for its wooden dialogue and stilted speeches, the gore-filled action sequences, had been considered - the same issue of past perfect passive.
C. ill-known at the time by its wooden dialogue and stilted speeches, gore-filled action sequences, is considered - We need 2nd conditional.
D. ill-known at the time for its wooden dialogue and stilted speeches and the gore-filled action sequences, would be considered - ok
E. which was known at the time in its wooden dialogue and stilted speeches and the gore-filled action sequences, would be considered - we need "known for."